The present invention relates to a termination structure for a semi-conductor device. In particular, the invention relates to the provision of a channel stop to prevent undesirable surface leakage between the drain and source of a transistor device, such as a MOSFET, which has a trench gate structure.
Trench MOSFETS are a well known form of MOSFET in which the gate electrode is formed in a trench in a semiconductor substrate. Application of an appropriate potential to the gate is used to establish a conducting channel between source and drain regions. It is also well known to provide a channel stop implant in the termination region of MOSFETS and similar devices to prevent undesirable surface leakage occurring due to unintentional channel formation.
Conventional channel stop structures comprise a heavily doped region provided in the termination region at the edge of the device to overcome doping depletion which could result in the leakage channel formation. Such structures are effective but have a disadvantage in that at least one additional masking step is required during the device fabrication to form the channel stop region.
One proposed solution to the above problem is disclosed in U.S. Pat. No. 5,614,751. This discloses a power MOSFET with a termination region provided with a channel stop comprising a trench lined with a thin layer of insulating oxide and filled with a conductive filler, such as polysilicon. The channel stop trench is formed simultaneously with the formation of trenches in the active area thus avoiding the need to provide a separate masking operation for formation of the channel stop. This trench channel stop structure, however, does not function as effectively as conventional implanted channel stop regions. The structure can be improved somewhat with the addition of an unmasked p-type implant operation—however this approach increases processing costs, and is also subject to the limitation that this applied p-type doping must necessarily be light with respect to the n-type doping level of the implanted body region. This consideration limits the effectiveness of the channel stop function provided by the p-type doped region in this structure.
It is an object of the present invention to obviate or mitigate the above disadvantages.
According to a first aspect of the present invention there is provided a semiconductor device comprising active and termination regions formed in a semiconductor substrate having an upper surface, a termination including a trench extending into the substrate from said upper surface within said termination region, wherein said termination trench is at least partly filled with an insulating material which extends from the termination trench to overlie adjacent regions of the device above said surface, and wherein a channel stop region extends laterally from a side wall of the termination trench into the substrate.
The structure according to the present invention includes a diffused or implanted channel stop which functions as effectively as conventional diffused channel stops, but which may be formed as parts of the active region are formed without the necessity for a dedicated masking or doping operation.
According to a second aspect of the present invention there is provided a method for forming a semiconductor device as defined above having active and termination regions, the method comprising providing a semiconductor substrate of a first conductive type and having a upper surface, forming a termination trench in the surface of said substrate in said termination region, and introducing a dopant into said substrate through at least a portion of a side wall of said termination trench to form said channel stop region.
Further preferred and advantageous features of the various aspect of the present invention will become apparent from the following description.